Method of optimizing channel characteristics using multiple masks to form laterally crystallized ELA poly-Si films

ABSTRACT

A method is provided to optimize the channel characteristics of thin film transistors (TFTs) on polysilicon films. The method is well suited to the production of TFTs for use as drivers on liquid crystal display devices. The method is also well suited to the production of other devices using polysilicon films. Regions of polycrystalline silicon can be formed with different predominant crystal orientations. These crystal orientations can be selected to match the desired TFT channel orientations for different areas of the device. The crystal orientations are selected by selecting different mask patterns for each of the desired crystal orientation. The mask patterns are used in connection with lateral crystallization ELA processes to crystallize deposited amorphous silicon films.

CROSS-REFERENCES

The subject matter of this application is related to the applicationentitled Method of Optimizing Channel Characteristics usingLaterally-Crystallized ELA Poly-Si Films by inventors Apostolos Voutsas,John W. Hartzell and Yukihiko Nakata filed on the same date as thisapplication, application Ser. No. 09/774,296.

The subject matter of this application is also related to theapplication entitled Mask Pattern Design to Improve Quality Uniformityin Lateral Laser Crystallized Poly-Si films by inventor ApostolosVoutsas filed on the same date as this application, application Ser. No.09/774,270.

All of these applications, which are not admitted to be prior art withrespect to the present invention by their mention here, are incorporatedherein by this reference.

BACKGROUND OF THE INVENTION

This invention relates generally to semiconductor technology and moreparticularly to a method of forming polycrystalline silicon within anamorphous silicon film.

Polycrystalline silicon thin film transistors (TFTs) can be used in avariety of microelectronics applications, especially active matrixliquid crystal displays (LCDs).

Thin film transistors (TFTs) used in liquid crystal displays (LCDs) orflat panel displays of the active matrix display type are fabricated onsilicon films deposited on a transparent substrate. The most widely usedsubstrate is glass. Amorphous silicon is readily deposited on glass.Amorphous silicon limits the quality of TFTs that can be formed. Ifdriver circuits and other components are to be formed on the displaypanel, as well as switches associated with each pixel, crystallinesilicon is preferred.

Silicon is often referred to as either amorphous or crystalline,including single crystal silicon. The term crystalline silicon can referto either single crystal silicon, polycrystalline silicon, or in somecases materials with significant quantities of micro-crystal structures.For many application, single crystal material is most desirable. But,single crystal silicon is not readily producible. Amorphous silicon canbe crystallized to form crystalline silicon by solid-phasecrystallization. Solid-phase crystallization is carried out by hightemperature annealing. But, glass substrates cannot withstand thetemperatures necessary to melt and crystallize silicon. Quartzsubstrates can withstand high temperature annealing, but quartzsubstrates are too expensive for most LCD applications.

Because glass deforms when exposed to temperatures above 600° C.,low-temperature crystallization (preferably below 550° C.) is used forsolid-phase processing of silicon on glass. The low-temperature processrequires long anneal times (at least several hours). Such processing isinefficient and yields polycrystalline silicon TFTs that have relativelylow field effect mobility and poor transfer characteristics.Polycrystalline silicon produced by solid-phase crystallization ofas-deposited amorphous silicon on glass suffers due to small crystalsize and a high density of intragrain defects in the crystallinestructure.

Excimer laser annealing (ELA) has been actively investigated as analternative to low-temperature solid-phase crystallization of amorphoussilicon on glass. In excimer laser annealing, a high-energy pulsed laserdirects laser radiation at selected regions of the target film, exposingthe silicon to very high temperatures for short durations. Typically,each laser pulse covers only a small area (several millimeters indiameter) and the substrate or laser is stepped through an exposurepattern of overlapping exposures, as is known in the art.

Lateral crystallization by excimer laser annealing (LC-ELA) is onemethod that has been used to form high quality polycrystalline filmshaving large and uniform grains. LC-ELA also provides controlled grainboundary location.

According to one method of conducting LC-ELA, an initially amorphoussilicon film is irradiated by a very narrow laser beamlet, typically 3-5micrometers wide. Passing a laser beam through a mask that has slitsforms the beamlet, which is projected onto the surface of the siliconfilm.

The beamlet crystallizes the amorphous silicon in its vicinity formingone or more crystals. The crystals grow within the area irradiated bythe beamlet. The crystals grow primarily inward from edges of theirradiated area toward the center. The distance the crystal grows, whichis also referred to as the lateral growth length, is a function of theamorphous silicon film thickness and the substrate temperature. Typicallateral growth lengths for 50 nm films is approximately 1.2 micrometers.After an initial beamlet has crystallized a portion of the amorphoussilicon, a second beamlet is directed at the silicon film at a locationless than half the lateral growth length from the previous beamlet.Moving either the laser, along with its associated optics, or by movingthe silicon substrate, typically using a stepper, changes the locationof the beamlet. Stepping a small amount at a time and irradiating thesilicon film causes crystal grains to grow laterally from the crystalseeds of the poly-Si material formed in the previous step. This achieveslateral pulling of the crystals in a manner similar tozone-melting-crystallization (ZMR) methods or other similar processes.

As a result of this lateral growth, the crystals produced tend to attainhigh quality along the direction of the advancing beamlets, alsoreferred to as the “pulling direction.” However, the elongated crystalgrains produced are separated by grain boundaries that run approximatelyparallel to the long grain axes, which are generally perpendicular tothe length of the narrow beamlet.

When this poly-Si material is used to fabricate electronic devices, thetotal resistance to carrier transport is affected by the combination ofbarriers that a carrier has to cross as it travels under the influenceof a given potential. Due to the additional number of grain boundariesthat are crossed when the carrier travels in a direction perpendicularto the long grain axes of the poly-Si material, the carrier willexperience higher resistance as compared to the carrier travelingparallel to the long grain axes. Therefore, the performance of TFTsfabricated on poly-Si films formed using LC-ELA will depend upon theorientation of the TFT channel relative to the long grain axes, whichcorresponds to the main growth direction. Typically, TFT performancevaries by a factor of between 2 and 4 as a function of orientationrelative to the main growth direction.

This difference in performance is undesirable from the point of viewthat as LCD resolution increases, or as panel size decreases, sizelimitations make it more desirable to have column drivers and rowdrivers oriented at ninety degrees relative to each other. Potentiallyresulting in one set of drivers having significantly differentcharacteristics relative to the other.

SUMMARY OF THE INVENTION

Accordingly, a method of forming polycrystalline regions on a substrateis provided. A first mask pattern is selected. A laser beam is directedthrough the first mask pattern to irradiate the substrate over aninitial region on the substrate. The region is annealed using a lateralcrystallization process. A second mask pattern is selected. The laserbeam is directed through the second mask pattern to irradiate thesubstrate over a second region on the substrate. The region is annealedusing a lateral crystallization process. If the first and second maskpattern have different orientations, the first region will have adifferent crystal orientation than the second region followingannealing.

The method of the present invention is well suited for producing devicesusing polycrystalline silicon. One application would be producing drivercircuits for LCDs. In which case, an LCD substrate, which can becomposed of quartz, glass, plastic or other suitable transparentmaterial, is used. An amorphous semiconductor material is deposited onthe LCD substrate to form a thin layer of amorphous silicon. Preferablythe semiconductor material will be silicon. A first region of theamorphous silicon is annealed using a first mask pattern in connectionwith a lateral crystallization ELA process to form a firstpolycrystalline region having elongated grain structures with a firstcrystal orientation. A second region of the amorphous silicon isannealed using a second mask pattern in connection with a lateralcrystallization ELA process to form a second polycrystalline regionhaving elongated grain structures with a second crystal orientation. Thesecond crystal orientation is different from the first crystalorientation, and preferably the crystal orientations are substantiallyninety degrees with respect to each other.

For certain applications it is desirable to form thin film transistors(TFTs) using the polycrystalline material formed by laser annealing. Ina preferred embodiment of the present method, TFTs having a firstchannel orientation are formed over the region with the first crystalorientation. The channel orientation is preferably substantiallyparallel to the crystal orientation, whereby the fewest number ofcrystal grain boundaries are crossed by the channel. TFTs having asecond channel orientation formed over the region with the secondcrystal orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an ELA apparatus usedin connection with the present method.

FIG. 2 shows mask patterns.

FIG. 3 illustrates a step in the process of lateral crystallizationusing ELA.

FIG. 4 illustrates a step in the process of lateral crystallizationusing ELA.

FIG. 5 illustrates a step in the process of lateral crystallizationusing ELA.

FIG. 6 is a flowchart diagram of an embodiment of the present invention.

FIG. 7 illustrates the formation of a substrate with multiple regions ofdifferent crystal orientation.

FIG. 8 illustrates the formation of TFTs with channels aligned to thecrystal orientation to optimize performance.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 a lateral crystallization excimer laser annealing(LC-ELA) apparatus 10 is shown. LC-ELA apparatus 10 has a laser source12. Laser source 12 may include a laser (not shown) along with optics,including mirrors and lens, which shape a laser beam 14 (shown by dottedlines) and direct it toward a substrate 16, which is supported by astage 17. The laser beam 14 passes through a mask 18 supported by a maskholder 20. The laser beam 14 preferably has an output energy in therange of 0.8 to 1 Joule when the mask 18 is 50 mm×50 mm. Currentlyavailable commercial lasers such as Lambda Steel 1000 can achieve thisoutput. As the power of available lasers increases, the energy of thelaser beam 14 will be able to be higher, and the mask size will be ableto increase as well. After passing through the mask 18, the laser beam14 passes through demagnification optics 22 (shown schematically). Thedemagnification optics 22 reduce the size of the laser beam reducing thesize of any image produced after passing through the mask 18, andsimultaneously increasing the intensity of the optical energy strikingthe substrate 16 at a desired location 24. The demagnification istypically on the order of between 3× and 7× reduction, preferably a 5×reduction, in image size. For a 5× reduction the image of the mask 18striking the surface at the location 24 has 25 times less total areathan the mask, correspondingly increasing the energy density of thelaser beam 14 at the location 24.

The stage 17 is preferably a precision x-y stage that can accuratelyposition the substrate 16 under the beam 14. The stage 17 is preferablycapable of motion along the z-axis, enabling it to move up and down toassist in focusing or defocusing the image of the mask 18 produced bythe laser beam 14 at the location 24.

The mask holder 20 is preferably capable of x-y movement. As shown, themask holder is holding two mask patterns. The two mask patterns could beformed on a single mask, or could be two separate masks. It is entirelypossible, and within the scope of the present invention, to have morethan two mask patterns. The mask holders x-y movement can be used toposition the desired mask pattern under the laser beam 14. The maskpatterns can be laid out linearly as shown. Or in the case of more thantwo mask patterns, the mask patterns can be arranged in a twodimensional array.

FIG. 2 shows a mask 18 having a first mask pattern 26 and a second maskpattern 27. The first mask pattern 26 comprises a first set of slits 29with a first slit spacing 30. The second mask pattern 27 comprises asecond set of slits 31 with a second slit spacing 32. In a preferredembodiment, the second mask pattern 27 corresponds to the first maskpattern 26 rotated by a desired angle. In a preferred embodiment, thedesired angle is substantially ninety degrees. As shown both maskpatterns are formed on a single mask, an alternative embodiment woulduse multiple masks to provide multiple patterns.

FIGS. 3 through 5 show the sequence of lateral crystallization employedas a portion of the present method. A region 34 of amorphous orpolycrystalline silicon overlies the substrate. The rectangular area 36corresponds to an image of one of the slits 30 projected on thesubstrate 16. The dashed line 38 corresponds to the centerline of theimage of the opening on the substrate.

FIG. 3 shows the region 34 just prior to crystallization. A laser pulseis directed at the rectangular area 36 causing the amorphous silicon tocrystallize. After each pulse the image of the opening is advanced by anamount not greater than half the lateral crystal growth distance. Asubsequent pulse is then directed at the new area. By advancing theimage of the slits 30 a small distance the crystals produced bypreceding steps act as seed crystals for subsequent crystallization ofadjacent material. By repeating the process of advancing the image ofthe slits and firing short pulses the crystal is effectively pulled inthe direction of the slits movement.

FIG. 4 shows the region 34 after several pulses. As is clearly shown,the area 40 that has already been treated has formed elongated crystalsthat have grown in a direction substantially perpendicular to the lengthof the slit. Substantially perpendicular means that a majority of linesformed by crystal boundaries 42 could be extended to intersect withdashed line 38.

FIG. 5 shows the region 34 after several additional pulses followingFIG. 4. The crystals have continued to grow in the direction of theslits' movement to form a polycrystalline region. The slits willpreferably continue to advance a distance substantially equal to adistance on the substrate corresponding to the slit spacing 32. Eachslit will preferably advance until it reaches the edge of apolycrystalline region formed by the slit immediately preceding it.

Referring now to FIG. 6, a flow chart of the steps of the method of thepresent invention is shown. Step 110 selects a first mask pattern.

Step 120 performs lateral crystallization using excimer laser annealing(ELA) to produce a polycrystalline region having a first crystalorientation. A laser beam is used to project an image of the mask ontothe substrate. The laser beam energy is sufficient to cause amorphoussilicon to crystallize. As discussed above a sequence of laser pulsescan be used to crystallize a region with a first crystal orientation.

Step 130 selects a second mask pattern. This second mask pattern ispreferably identical to the first mask pattern rotated to a differentangle. Preferably the different angle is substantially perpendicular tothe first pattern.

Step 140 performs lateral crystallization ELA to produce apolycrystalline region having a second crystal orientation. The secondcrystal orientation is preferably substantially perpendicular to thefirst crystal orientation.

The steps of selecting the first or second mask pattern (steps 110 and130) can be accomplished using a mask holder capable of repositioningthe mask to orient a desired pattern under the laser beam 14. The maskpatterns can be on a single mask, or each mask pattern can be located ondifferent individual masks. Although, we have described the method ofthe present invention in terms of using two mask patterns, in someapplications it may be desirable to use three or more patterns. Ifadditional patterns are used the mask holder will preferably be capableof selecting from multiple mask patterns. Preferably, the mask holderselects masks by using x-y motion to reposition the mask holder underthe laser beam 14.

In performance of the method, if multiple regions of the sameorientation are desired, it is preferable to produce all of the regionswith the first crystal orientation prior to changing the mask andproducing regions of the second crystal orientation. Multiple regionswith the same orientation are preferred when producing multiple deviceson a single substrate.

FIG. 7 shows the substrate 16 with two display regions 210 and 220. Eachdisplay region corresponds to the location of a final LCD or otherdisplay device. The first mask is selected. Then the image 222 of thefirst mask is projected at a first starting position 224.

In an embodiment of the present method, the image 222 is moved one stepat a time by moving the mask stage. At each step a laser pulsecrystallizes a portion of the silicon material. Once the image 222 hasmoved a distance corresponding to the slit spacing, the substrate ismoved to position the image 222 over an adjacent position 226. The maskstage is then moved to crystallize the underlying region. By repeatingthis process across the substrate, a line of polycrystalline materialhaving predominantly a first crystal orientation is formed. The image222 is repositioned at a position corresponding the start of theadjacent uncrystallized region. The process is repeated until a region230 is formed having predominantly a first crystal orientation. As shownthis orientation is horizontal. After a first region 230 is formed,repeating the process discussed above can produce a second region 240having the same general crystal orientation as the first region 230.

In a preferred embodiment, once regions of a first crystal orientationhave been produced, the second mask is selected. The second maskpreferably has a pattern that is substantially perpendicular to thefirst mask pattern. The process is then repeated to produce regions witha second crystal orientation. Preferably, the second crystal orientationwill be substantially perpendicular to the first crystal orientation. Athird region 250 is formed by positioning the second mask image 245 overanother starting point and processing the region as discussed aboveuntil the region 250 has been crystallized. A fourth region 260 couldthen be crystallized to have the same orientation as the third region250.

In this manner, multiple regions can be crystallized with two or morecrystal orientations. The order of crystallization is not critical tothe present invention.

Once the substrate 16 has been processed to form regions with thedesired crystal orientation, device elements are formed on the substrateas illustrated in FIG. 8. FIG. 8 is for illustration purposes, and aswith the other drawings, is not drawn to scale. The substrate 16 has afirst polycrystalline region 330 and a second polycrystalline region 340with the same crystal orientation. A first set of TFTs 345 have beenformed within polycrystalline regions 330 and 340. First set of TFTs 345have channels 347 oriented to match the crystal orientation of theunderlying regions 330 and 340. As shown in the figure, both the crystalorientation of regions 330 and 340, and the channels 347 are horizontal.Third polycrystalline region 350 and fourth polycrystalline region 360are shown having a crystal orientation substantially perpendicular tothe crystal orientation of regions 330 and 340. A second set of TFTs 365having channels 367 are substantially perpendicular to the first set ofTFTs 345 and channels 347, and substantially parallel to the crystalorientation of the underlying regions 350 and 360.

Since FIG. 8 illustrates a display device, pixel regions 370 are shown.The pixel regions 370 can have the same underlying crystal orientationas either the regions under the first set of TFTs 345, also referred toas row drivers, or the second set of TFTs 360, also referred to as thecolumn drivers. As shown in FIG. 8, the pixel region is matched to thecolumn drivers. If the substrate shown in FIG. 7 were used, the pixelregion would match the row drivers. For some applications, it may not benecessary to crystallize the entire substrate. Some regions may not needto be crystallized including, but not limited to the pixel regions.

Although the present method is well suited to producing display devices,it is also suited to other types of device produced using apolycrystalline material produced on an underlying substrate. Inaddition to row and column drivers, other circuitry unrelated todisplays can be produced.

The terms perpendicular and parallel should not be construed narrowly tolimit the scope of the present method, especially in reference tocrystal orientation. The terms substantially perpendicular andsubstantially parallel should be construed broadly. A broader definitionof these term parallel is therefore provided. If a feature, orstructure, is said to be parallel to the crystal orientation, thestructure crosses the fewest crystal grain boundaries in the relevantdirection.

Several embodiments of the method of the present invention have beendescribed. Variations on these embodiments will be readily ascertainableby one of ordinary skill in the art. Therefore, the description here isfor illustration purposes only and should not be used to narrow thescope of the invention, which is defined by the claims as interpreted bythe rules of patent claim construction.

What is claimed is:
 1. A method of forming polycrystalline regions on asubstrate comprising the steps of: a) selecting a first mask pattern; b)directing a laser beam through the first mask pattern to irradiate thesubstrate over an initial region on the substrate; c) annealing theinitial region using a lateral crystallization process; d) selecting asecond mask pattern; e) directing the laser beam through the second maskpattern to irradiate the substrate over a second region on thesubstrate; and f) annealing the second region using a lateralcrystallization process.
 2. The method of claim 1, wherein the firstmask pattern is a plurality of parallel slits at a first orientation. 3.The method of claim 2, wherein the second mask pattern is a plurality ofparallel slits at a second orientation.
 4. The method of claim 3,wherein the first orientation and the second orientation areapproximately ninety degrees relative to each other.
 5. The method ofclaim 1, wherein the mask is mounted to a mask holder, which isselectable to allow a mask pattern to be selected.
 6. A method ofprocessing a substrate comprising the steps of: a) depositing amorphoussilicon on a substrate; b) annealing a first region on the substrateusing a first mask pattern in connection with a lateral crystallizationELA process to form a first polycrystalline region having elongatedgrain structures with a first orientation; c) annealing a second regionon the substrate using a second mask pattern in connection with alateral crystallization ELA process to form a second polycrystallineregion having elongated grain structures with a second orientation,which is different from the first orientation; d) forming a first TFThaving a channel oriented substantially parallel to the elongated grainstructures of the first polycrystalline region; and e) forming a secondTFT having a channel oriented substantially parallel to the elongatedgrain structures of the second polycrystalline region.
 7. The method ofclaim 6, wherein the substrate is a transparent material.
 8. The methodof claim 6, wherein the substrate is quartz, glass, or plastic.
 9. Themethod of claim 6, wherein the second orientation is substantiallyperpendicular to the first orientation.
 10. The method of claim 6,wherein the first mask pattern and the second mask pattern are formed onthe same mask.
 11. The method of claim 6, wherein the mask is mounted toa mask holder, which is capable of selecting from a plurality of maskpatterns.
 12. The method of claim 6, wherein the mask is mounted to amask holder, which is capable of selecting from a plurality of masks.13. The method of claim 6, wherein the first mask pattern is formed on afirst mask.
 14. The method of claim 6, wherein the second mask patternis formed on a second mask.
 15. A method of processing an LCD substratecomprising the steps of: a) depositing amorphous silicon on a substrate;b) selecting a first mask pattern; c) forming row drivers by annealing afirst plurality of regions on the substrate using a lateralcrystallization ELA process to form polycrystalline regions havingelongated grain structures with a first orientation and forming a firstplurality of TFT structures having channels oriented substantiallyparallel to the elongated grain structures with the first orientation;d) selecting a second mask pattern; and e) forming column drivers byannealing a second plurality of regions on the substrate using a lateralcrystallization ELA process to form polycrystalline regions havingelongated grain structures with a second orientation, which is differentfrom the first orientation and forming a second plurality of TFTstructures having a channel oriented substantially parallel to theelongated grain structures of the second polycrystalline region.
 16. Themethod of claim 15, wherein the second orientation is substantiallyperpendicular to the first orientation.
 17. The method of claim 15,further comprising forming a pixel region.
 18. The method of claim 17,wherein the pixel region is formed by annealing a region on thesubstrate using a lateral crystallization ELA process to formpolycrystalline regions having elongated grain structures with the firstorientation.
 19. The method of claim 17, wherein the pixel region isformed by annealing a region on the substrate using a lateralcrystallization ELA process to form polycrystalline regions havingelongated grain structures with the second orientation.
 20. The methodof claim 1, wherein the step of annealing the initial region furthercomprises exposing the initial region to a laser pulse, wherebyamorphous silicon is crystallized, and advancing the first mask patternto a position not greater than half the lateral crystal growth distanceand exposing the initial region to a subsequent laser pulse, whereby thecrystals produced by the laser pulse act as seed crystals for thecrystallization of the amorphous silicon exposed to the subsequent laserpulse.
 21. The method of claim 6, wherein the step of annealing thefirst region further comprises exposing the first region to a laserpulse, whereby amorphous silicon is crystallized and advancing the firstmask pattern to a position not greater than half the lateral crystalgrowth distance and exposing the first region to a subsequent laserpulse, whereby the crystals produced by the laser pulse act as seedcrystals for the crystallization of the amorphous silicon exposed to thesubsequent laser pulse.